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UCC28950: controller runs in CCM mode under all conditions.

user6004348
Intellectual 390 points
Part Number: UCC28950

HI SIR

Because of the light load EMI problem, I tried to operate the IC in full CCM mode. I would like to know what special issues need to be paid attention to when operating in full CCM mode?

In addition, I changed a prototype from DCM to full CCM, and the SR MOS was damaged at 24V/0.2A (800W). Will this problem occur in full CCM mode?

  • +1
    TI__Guru* 85926 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly

    Regards,

  • +1 in reply to Mike O'
    TI__Guru* 85926 points

    Hello,

    Continues Conduction Mode refers to the output inductor current being continuous.  However, in all designs there is a point were the design will go discontinuous.  The point where the design will go discontinues is when the output current (Iout) is less than half the inductor ripple current.

    Iout <  dIL/2 

    However when using synchronous rectifiers (SRs) the design can always be CCM.  This is was the data sheet was referring too.  However, if you are using SRs I would recommend using the DCM pin and disabling for the FETs have a reverse current going through them.  Reverse currents in FETs have a tendency to damage them when they turn off.  The following link will bring you to an application note that shows how to design with the UCC28950 in a PSFB and how to setup the DCM comparator.  https://www.ti.com/lit/an/slua560d/slua560d.pdf

    This topology is ZVS switching in the primary so it should not be contributing much to EMI.  To reduce EMI you could consider putting an RC snubber network across the output rectifiers to dampen the ringing at these switch nodes.   This would help reduce EMI.

    The other thing that will help is proper EMI filtering.  The following link will bring you to training material on how to setup and common mode and differential mode filters.  This should help you in trying to get your design to pass EMI.

    https://www.ti.com/lit/ml/slyp757/slyp757.pdf

    Regards,